In northern climates, and particularly in locations where there is intermitent freezing and thawing, snow which accumulates on the roof will melt and flow downwardly along the roof toward the eaves or overhang where the attic heat is no longer present to assist melting. Due to the thermal drop in the area of the eaves, the melted snow freezes to form ice packs that block further run off and act as dams so that subsequently melted snow will seep under the shingles and through nail holes or sheathing cracks. The seeping water can cause staining or discoloration of the ceiling and walls of the building.
The formation of ice packs at the eaves can be prevented by eliminating the thermal gradient and providing a free flow of air from the eaves to the ridge of the roof. It has been found that the key region where ventilation and thermal protection is most critical is the restricted vertical clearance between the top of the plate of the vertical wall and the roof deck. In normal building construction, the ceiling joists and rafters are nailed to the top plate of the vertical wall and usually a heel cut in the rafters reduces the vertical clearance between the top plate and the roof deck to as little as an inch or two.
Furthermore, the ceiling of the building is normally insulated by use of either blown insulation or insulation batts and very often the insulation is stuffed or blown into this restricted vertical clearance between the top plate and the roof deck, thus reducing or entirely eliminating the free flow of air along the underside of the roof deck. On the other hand, if full insulation is not carried through along the ceiling to the top plate, the area of the ceiling adjacent the top plate will be inadequately insulated, resulting in cold spots and possible condensation on the ceiling.
U.S. Pat. No. 3,683,785, is directed to a roof construction utilizing baffles which are located between adjacent rafters and are spaced from the undersurface of the roof deck to define air channels that extend from the facia to a location inwardly of the exterior wall of the building.
The U.S. patent application Ser. No. 272,380 filed July 17, 1972 now U.S. Pat. No. 3,797,180 patented Mar. 19, 1974 also discloses a construction for providing air flow along the underside of the roof. In that application a sheet of corrugated material is nailed to the upper surface of the rafters before applying the roof deck, with the sheet material bridging the top plate of the exterior wall. The corrugated sheet material, in combination with the roof deck, defines channels for the flow of air along the undersurface of the roof deck.